The present invention relates to an access control system for rotating an information storage disc at a variable angular velocity or at a constant linear velocity.
An example of such a disc of a constant linear velocity type is disclosed in U.S. Pat. No. 4,223,349 and examples of control systems are disclosed in the U.S. Pat. No. 4,223,349 and Japanese laid open Patent No. 193167/1985. The access control system disclosed in the Japanese laid open Patent includes, in order to move compulsively the optical transducer in the radius direction, a pickup location detector for detecting a location of an optical transducer on the radius axis, the optical transducer being movable in the radius direction to pickup the information from the rotating disc, and a divider circuit for producing a rotation number in response to a detection signal from the pickup location detector. Conventional divider circuits are complicated and highcost products. Further, a potentiometer whose slider terminal is coupled to the optical transducer is used for the pickup location detector, so it is required to be highly precise in assembling and adjusting the pickup location detector.